264 JOHN D. DODSON 



hunger but a sudden decrease from forty-one to forty-eight hours 

 of hunger. The first part of the curve needs no explanation, for 

 the difference in the rate of habit formation when a subject is 

 putting its whole energy in the accomplishment of the act and 

 when it is more or less indifferent towards the performance is 

 fairly well established. Rats which pass from the door between 

 the entrance chamber and nest box in sixty-six hundredths of a 

 second were making about the greatest speed possible for such 

 animals. The scratching reflex or any other distracting in- 

 fluence seldom interfered with their choosing. But the rats 

 which took one and five-tenths seconds had time to scratch occa- 

 sionally or explore the entrance chamber. But the cause of the 

 rapid decrease in the rate of learning for animals of forty-eight 

 hours of hunger is not so apparent. Nor can this decrease be 

 accounted for in terms of poor physical condition of the subjects. 

 At the end of the series of experiments all these animals were in 

 good physical condition though they had somewhat less flesh 

 than their mates which had been trained with electric shock. 



Had one who did not know their physical condition been 

 observing the manner of choice of this group of rats he would 

 have immediately come to the conclusion that they were not 

 hungry. They neither rushed to get food nor ate eagerly when 

 they had reached it. Their, behavior was very much the same as 

 that of subjects which had gone for only twenty-four hours with- 

 out food. It is true that the carriage of the rat was somewhat 

 different. Dr. J. A. Carlson's careful experiments on hunger in 

 man and dogs account for the behavior of this group of animals 

 in a most satisfactory manner ( 1 ) . Dr. Carlson has demonstrated 

 rather conclusively that the sense of hunger is due to " certain 

 types of contractions in the empty or nearly empty stomach. 

 That these contractions stimulate nerves in the sub-mucosa or 

 muscularis." He demonstrated experimentally that these con- 

 tractions persist almost constantly after the first day of hunger 

 in man : and in young dogs until a short time before death from 

 starvation. In describing his own experience for a period of five 

 days of hunger he says, 



